In a coating area, a top coat is usually applied over an intercoat (or underlying coat) at complete hiding, i.e. to cover up the same. At this time, since the hiding power differs considerably from pigment to pigment, the actual top coat thickness likewise varies in a considerable range from coating to coating. If a low hiding paint composition whose hiding power is in the order of about 50.about.250.mu. as expressed herein in terms of film thickness required for hiding the JIS contrast chart (reflectance of white substrate: Rw=80.+-.1%; reflectance of black substrate: Rb.ltoreq.2%) by visual judgement, is to be applied as a top coat, repeated and somewhat complicated coating operations are always required. This makes for a very time consuming procedure and hence is not practicable. Even in the case of using a higher hiding paint composition, if the substrate is of a complicated structure and there are hidden portions which make it difficult to coat the substrate, it is not possible to cover up the whole area equally and there often results a lack of hiding from portion to portion. If one wishes to have a complete hiding, then there results the problem of sagging at the other portions because of the excessive coating applied thereupon. Therefore, it is very difficult to hide thoroughly the intercoat with a normal application thickness of 30 to 40.mu.. On the other hand, a highly chromatic coating has been welcomed in the market in recent years. Since the most organic pigments used in such coating have a fairly low hiding power, considerable quantities of such pigment must be added to the top coat composition to result in a complete hiding of the intercoat and a film thickness of about 30.about.40.mu. is required. Employment of such a larger quantities of pigment, however, may cause additional problems, under normal conditions, in that the painted film lacks gloss, the coating composition is highly viscose and the film exhibits poor weather resistance. Therefore, in such a case one cannot help but use coatings such that they incompletely hide the intercoating. For this reason, applicants are forced to select an intercoat which is the same or somewhat similar to a given colored top coat. However, there is no established standard for selecting such intercoats and therefore, at the time when different kinds of pigments are to be used in the intercoat and in the top coat, one must necessarily rely on a trial and error system for the determination of an appropriate color combination. Such a working procedure is indeed time consuming and inefficient. Furthermore, since the colored intercoat does include a higher concentration of pigment of the same, or substantially same hue as that of the top coat, there is a serious problem of raising up the cost as a matter of course. Therefore, if an appropriate intercoat color, which may give as small color difference between the composite coating having a low hiding top coat applied at 30.about.40.mu. thickness, i.e. at incomplete hiding, over the intercoat and a coating having the same top coat applied under complete hiding conditions, can be easily determined from a wider range of colors, it would be extremely beneficial in widening the color gamut of the top coat. This would open the way for using low hiding pigments previously thought to be unsuitable for use in a top coat where no adequate color for the intercoat has previously existed. The selection of such top coat would also shorten the time for selecting an appropriate intercoat color for a given top coat, thus lowering the cost of the intercoats and top coats due to the decrease in the amount of pigment and also in shortening the application time and the like.
The inventors have previously found a method of forming a highly chromatic multi-layer colored coating of red (Munsell renotation hue R series) yellow (Munsell renotation hue Y series) or orange (Munsell renotation hue YR series) comprising applying over a colored intercoat a low hiding colored top coat at incomplete hiding but which top coat has a color difference of less than 1.0 as compared with the color of the same top coat applied at complete hiding. This is achieved by selecting the intercoat color so as to fulfill the requirement: EQU .SIGMA..vertline..DELTA.R.lambda..vertline..ltoreq.0.5
wherein .SIGMA..vertline..DELTA.R.lambda..vertline. stands for the summation of absolute values of .DELTA.R.lambda., and .DELTA.R.lambda. is the differentia between the spectral reflectance (RM.lambda.) of the top coat applied at complete hiding and the spectral reflectance (RG.lambda.) of the intercoat applied at complete hiding at the respective wavelength (.lambda.) at intervals of 20 nm in the selected wavelength region ranging from the point, at which the reflectance curves over the white and the black substrates of the JIS contrast chart of the top coat applied at incomplete hiding and in a defined film thickness, is more than 1% to 700 nm. According to this method it is also required that the reflectance curves of the complete hiding top coat and of the complete hiding intercoat come approximately near to each other only in the abovementioned selected wavelength region. Applicants have applied for a patent for this invention in Japanese Patent Application No. 49632/82, on Mar. 27, 1982. As already stated, when different kinds of pigments are to be used in the intercoat and the top coat, no reliable and established method had been known for the selection of an appropriate intercoat color which might be the same or quite similar color to that of the top coat, thus the intercoat was selected by trial and error, which is both time consuming and inefficient. The abovesaid was indeed a very important and useful invention in a sense that an appropriate intercoat color could be easily determined from a number of colors for a given top coat. However, since the selection of a colored intercoat similar to that of the top coat is the prerequisite of that invention, it is of course essential that the color be selected as close as possible to the top coat color. On the other hand, a pigment of a high hiding power is generally required for an intercoat. There are many cases wherein the composite structure of a low hiding top coat and a high hiding inter coat be required. At that time, since the both paints markedly differ from each other in pigment composition and in their compounding ratio, difficulties are apt to occur in the selection of optimum intercoat color. If it is possible to use an intercoat of universal color unrelated to the top coat color and obtain a multi-layer coating comprising a colored top coat applied over the intercoat under incomplete hiding conditions, while still maintaining a color similar to that of the top coat applied at complete hiding, it would be an inestimable advance of the art.
The inventors, for the purpose of determining the wavelength region in which the intercoat color will exert influence upon the top coat color, have applied a top coat at incomplete hiding and at a defined film thickness (30.about.40.mu.) over the white and the black substrates of a JIS contrast chart and examined the spectral reflectance (Rw.lambda.) over the white substrate and the spectral reflectance (RB.lambda.) over the black substrate, as well as the spectral reflectance (RM.lambda.) of the top coat applied at complete hiding, at the respective point at intervals of 20 nm in the wavelength region from 400 to 700 nm (visible wavelength region). The thus-obtained data were then plotted in the same chart to draw three spectral reflectance curves for three respective colors. These curves are shown in FIG. 1, in which (A) is for the R series (Munsell renotation hue, hereinafter the same) color, (B) is for the GY,G,BG or the B series color and (C) is for the PB series color, and in each drawing, the FIG. 1 represents the spectral reflectance curve of top coat applied at complete hiding and the FIGS. 2 and 3 are of the same top coats applied over the white and the black substrates at incomplete hiding, respectively. Though a color shows each different curve pattern, it is clear that in any case, the spectral reflectance curve (1) of the top coat applied at complete hiding is always between the curves (2) and (3) of the same top coats applied over the white and the black substrates at incomplete hiding. This clearly indicates that if one were to apply the top coat over the intercoat of a grayish color, (i.e. a mixed color of white and black) under incomplete hiding conditions, the corresponding spectral reflectance curve must be at a very close position to the spectral reflectance curve (1).
The invention has been made starting from the aforesaid theoretical studies.